1. Field of the Invention
The present invention generally relates to the field of pulse compression radar, and more specifically to a method and apparatus for integrating a plurality of compressed pulses which automatically compensates for variation of pulse repetition rate and interpulse jitter.
2. Description of the Related Art
Pulse compression is used advantageously in radar systems, including laser radar or "lidar" systems which operate at optical rather than radio frequencies. Pulse compression involves the transmission of a long coded pulse and the processing of the received echo to obtain a relatively narrow pulse. The increased detection capability of a long-pulse radar system is achieved while retaining the range resolution capability of a narrow-pulse system. Transmission of long pulses permits a more efficient use of the average power capability of the radar without generating high peak power signals. The average power of the radar may be increased without increasing the pulse repetition frequency (PRF) and thereby decreasing the unambiguous range of the radar.
In pulse compression radar, a long pulse is generated from a narrow pulse which contains a large number of frequency components with a precise phase relationship between them. The relative phases are changed by a phase-distorting filter such that the frequency components combine to produce a stretched, or expanded pulse which is then amplified and transmitted. In some classes of radar where the transmitted signal frequency is much greater than practical phase-distorting filters such as surface acoustic wave (SAW) devices can accommodate, the transmitter must be frequency modulated directly in order to produce an expanded pulse. This is the case for laser radar. The received echo is processed in the receiver by a compression filter, which readjusts the relative phases of the frequency components so that a narrow or compressed pulse is again produced.
Various pulse compression methods are known in the art, including linear frequency modulation (FM), non-linear FM and phase-coding. Linear FM pulse compression, also known as "chirp", is especially advantageous in that in addition to determining the range to a target, the relative doppler can be obtained simultaneously with resolution equivalent to that of long-pulse radar system.
Linear FM pulse compression is illustrated in FIG. 1. The transmitted pulses have a center carrier frequency F.sub.C. The frequency of the carrier is varied through a range .DELTA.f which is centered about F.sub.C over a duration or pulse width .DELTA.t of the pulses. The modulation or "chirp" slope of the pulses is therefore .DELTA.f/.DELTA.t. The time period between successive pulses is equal to 1/PRF and designated as .DELTA.T.
SAW devices are widely used as matched filters for pulse compression. Such a device includes an input transducer and an output transducer mounted on a piezoelectric acoustic substrate. The transducers are usually fabricated as interdigital structures by metal film deposition on the surface of the acoustic medium. The interdigital structures include fingers that dictate the frequency characteristic of the unit. The geometry of the fingers may also be designed to provide suppression of sidelobes which are generated outside the bandwidth of the expanded pulses.
Pulse compression radar and the use of SAW matched filters for pulse compression are known in the art per se, such as described in Chapter 10 of a textbook entitled "Radar Handbook, Second Edition", edited by M. Skolnik, McGraw-Hill 1990. A lidar system which utilizes a SAW device to perform linear FM pulse compression is disclosed in U.S. Pat. No. 4,666,295, entitled "LINEAR FM CHIRP LASER", issued May 19, 1987 to R. Duvall et al.
Detection of weak target returns can be enhanced using a technique known as "post detection integration", in which several consecutive received pulses are integrated or summed together to produce a composite pulse having an amplitude which is substantially larger than that of the individual pulses. This is accomplished by transmitting a burst of pulses, typically four, and feeding the corresponding received pulses through a SAW tapped delay line which is designed such that the pulses appearing at the taps are aligned with each other in time. The pulses are then compressed, envelope detected and summed together to provide a composite integrated pulse for subsequent processing.
A conventional radar system utilizing pulse compression and post detection integration is illustrated in FIG. 2 and generally designated as 10. A frequency modulator 12 modulates short pulses produced by a pulse generator 14 to produce long expanded pulses by, for example, linear FM modulation. The expanded pulses are fed through a transmitter 16 to an antenna 18 which radiates the expanded pulses toward a target (not shown) for detection. Received pulses which are reflected from the target are picked up by the antenna 18 and fed through the transmitter 16 to a mixer 20 in which the pulses are mixed with a signal from a local oscillator 22. The resulting signals are converted to an intermediate frequency (IF) and fed through a backscatter filter 24 to a SAW matched filter 26.
The filter 26 compresses the received pulses and feeds them to a post integration unit 28. The filter 26 is also provided with a weighting function which suppresses time sidelobes of the received pulses. The time sidelobes generated by compressing an expanded pulse with a compression SAW are analogous with the time sidelobes associated with computing a discrete Fourier transform with finite time series signals. The time sidelobes are typically reduced in amplitude from -13dBc to -40 dBc.
The pulses are transmitted in bursts which typically consist of many pulses. However, in this particular example we will consider the bursts to consist of four pulses transmitted at a PRF of 1/.DELTA.. The post detection integration unit 28 includes a SAW tapped delay line 30 which includes delay elements 30a, 30b and 30c. The compressed pulses from the matched filter 26 are fed into the unit 28 and split into first to fourth channels CH0 to CH3. The first channel CHO includes an envelope detector 32 which demodulates the compressed pulses applied directly thereto from the matched filter 26, designated as first compressed pulses, to produce first demodulated pulses which are fed through a low pass filter to a summer or integrator 36.
The delay elements 30a, 30b and 30c each delay the compressed pulses propagating therethrough by a time period equal to .DELTA.T. The second channel includes an envelope detector 38 and low pass filter which receive second compressed pulses from a tap between the delay elements 30a and 30b and produce second demodulated pulses which are applied to the integrator 36. The second compressed pulses and second demodulated pulses are delayed by a time period .DELTA.T relative to the first compressed and demodulated pulses. The third and fourth channels CH2 and CH3 include envelope detectors 42 and 46 and low pass filters 44 and 48 respectively which are the same as those in the first and second channels CH0 and CH1. The third channel CH2 receives third compressed pulses from the tap between the delay elements 30b and 30c which are delayed by 2.DELTA.T, whereas the fourth channel CH3 receives fourth compressed pulses from the output of the delay element 30c which is delayed by 3.DELTA.T.
As illustrated in FIG. 2, the four pulses of the burst are designated as A, B, C and D respectively, and are summed by the integrator 36 at a time indicated as 50 at which the fourth pulse D emerges from the low pass filter 34 in the first channel CH0. Due to the delay of .DELTA.T through the delay element 30a, the third pulse C emerges from the low pass filter 40 in the second channel CH1 at the time 50. Similarly, due to the delay of 2.DELTA.T, the second pulse B in the third channel CH2 emerges from the low pass filter 44 at the time 50. The first pulse A emerges from the low pass filter 48 in the fourth channel CH3 at the time 50 due to the delay of 3.DELTA.T. As a result, the four pulses A, B, C and D of the burst are summed together or integrated by the integrator 36 to produce an integrated or composite pulse E having an amplitude which is substantially larger than that of the pulses A, B, C and D.
Although effective, the radar system 10 is only capable of operating at a single PRF. If the PRF were changed from .DELTA.T, the pulses A, B, C and D would no longer be aligned in time at the integrator 36 since the time period .DELTA.T would change although the propagation delay through the delay line 30 would remain constant. In addition, the time period .DELTA.T between pulses varies to a significant extent in actual radar equipment due to jitter caused by electronic noise and other factors. Such jitter also causes the pulses to be misaligned, resulting in reduced amplitude of the integrated pulse E and decreased range resolution.